This invention relates to a system for driving a gas discharge lamp and more specifically to a system employing pulse width modulation for reducing color segregation in high intensity gas discharge lamps.
High intensity discharge lamps (HID) are becoming increasingly popular because of their many advantages, such as efficiency and light intensity. These HID lamps are driven by either a high frequency electronic ballast that is configured to generate driving current signals at above 20 Khz range or by a low frequency electronic ballast with driving current signals in the 100 Hz range.
A major obstacle to the use of high frequency electronic ballasts for HID lamps, however, is the acoustic resonances/arc instabilities which can occur at high frequency operation. Acoustic resonances, at many instances, can cause flicker of the arc which is very annoying to humans. Furthermore, acoustic resonance can cause the discharge arc to extinguish, or even worse, stay permanently deflected against and damage the wall of the discharge lamp.
Recently, a new class of high intensity discharge lamps has been developed that employ ceramic (polycrystalline alumina) envelopes. The discharge envelope in this class of lamps is cylindrical in shape, and the aspect ratio, i.e., the inner length divided by the inner diameter is close to one, or in some instances more than one. Such lamps have the desirable property of higher efficacy, but they have the disadvantage of having different color properties in vertical and horizontal operation. In particular, in vertical operation color segregation occurs.
The color segregation can be observed by projecting an image of the arc onto a screen, which shows that the bottom part of the arc appears pink, while the top part appears blue or green.
This is caused by the absence of complete mixing of the metal additives in the discharge. In the upper part of the discharge there is excessive thallium emission and insufficient sodium emission. This phenomena leads to high color temperature and decreased efficacy.
U.S. application Ser. No. 09/335,020 entitled Reduction of Vertical Segregation In a Discharge Lamp, filed Jun. 17, 1999, now U.S. Pat. No. 6,184,633, and incorporated herein by reference, teaches a method to eliminate or substantially reduce acoustic resonance and color segregation, by providing a current signal frequency sweep within a sweep time, in combination with an amplitude modulated signal having a frequency referred to as second longitudinal mode frequency. The typical parameters for such operation are a current frequency sweep from 45 to 55 kHz within a sweep time of 10 milliseconds, a constant amplitude modulation frequency of 24.5 KHz and a modulation index of 0.24.
The modulation index is defined as (Vmaxxe2x88x92Vmin)/(Vmax+Vmin), where Vmax is the maximum peak to peak voltage of the amplitude modulated envelope and Vmin is the minimum peak to peak voltage of the amplitude modulated envelope. The frequency range of 45 to 55 KHz is between the first azimuthal acoustic resonance mode and the first radial acoustic resonance mode. The second longitudinal mode can be derived mathematically, where the power frequency of the nth longitudinal mode is equal to n*C1/2L where n is the mode number, C1 is the average speed of sound in the axial plane of the lamp and L is the inner length of the lamp.
In terms of setting up a lamp driving system, it is relatively convenient to provide an arrangement of signal generators in a laboratory environment to produce the desired waveform signal that includes amplitude modulation at the second longitudinal mode frequency and the current signal sweep frequency. However, there is a need for a practical driver configuration in a lamp power converter that provides the required driving signal to the lamp in a convenient and efficient manner.
Thus, in accordance with one embodiment of the invention, a high intensity discharge lamp is driven by an electronic ballast that includes a ballast bridge unit having a full-bridge or a half-bridge configuration, that is controlled by a pulse width modulated (PWM) signal generator. The pulse width modulated signal generator provides a pulse width modulated signal that is derived from a lamp drive voltage waveform having the desired characteristics as required for eliminating color segregation in the lamp. This signal controls the operation of the bridge. The output signal provided by the ballast bridge unit is then provided to a low pass filter, which is employed to filter the undesired frequency components generated by the pulse width modulated signal, as well as to ballast the lamp.
In accordance with another embodiment of the invention, a controller module is employed to first determine the second longitudinal mode frequency that corresponds to the second longitudinal mode of the discharge lamp and then to provide the pulse width modulated signal provided by the pulse width modulated signal generator. The controller module includes a microprocessor configured to receive feedback voltage and current signals from the driven high intensity discharge lamp. The controller also includes an AM/FM signal generator that provides the desired lamp drive voltage waveform. As described before, the lamp drive voltage waveform includes an amplitude modulated signal having a frequency equal to the second longitudinal mode frequency of the discharge lamp in combination with a frequency swept signal.
The lamp drive voltage waveform provided by the AM/FM signal generator is then fed to the pulse width modulated signal generator. When operating in the symmetrical mode, the pulse width modulated signal generator provides a symmetrical triangular pulse modulating signal that is compared with the drive voltage waveform via a comparator. When operating in the asymmetrical mode, the pulse width modulated signal generator provides a triangular signal having a one sided ramp. The output port of the pulse width modulated signal generator provides a gate drive signal to operate the bridge module.
In accordance with another embodiment of the invention all the components of the controller module are implemented in analog and/or digital circuit configurations. The pulse width modulated signal provided by the bridge circuit drives the lamp via a low pass filter circuit. Advantageously the resonance frequency of the low pass filter falls within a range above any of the frequency components of the voltage spectrum of the desired lamp drive waveform, and one third of the resonant frequency falls between the swept frequency range and the full range of the possible second longitudinal mode frequency range.